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April 14, 2008
Radiation (Chapter 2, pg. 31 forward, Continued)
 We are in the process of going through the supplemental reading material on radiation to help
describe what is meant by an electromagnetic wave, how are different types of radiation
distinguished, and several fundamental laws of radiation emission. I believe that the material
presented in the supplemental reading will be easier to understand than the material in the
textbook. We will begin Monday with the section “Rules Governing the Emission of
Radiation” in the supplemental pages.
o The supplemental reading page for electromagnetic radiation is available under the
reading link on these class web pages. I strongly suggest that you read over this
material before coming to class. It may also be helpful to print the pages so that you
have them in front of you when we discuss this material.
 Quick Summary
o Review figure showing electromagnetic spectrum (linked on lecture notes page)
o All objects in the universe emit (or give off) radiation energy. The amount and type
of radiation emitted depends strongly on the temperature of the object.
 The higher the temperature, the more total energy radiated
 The higher the temperature, the shorter the wavelength of the peak radiation
energy emission.
 Therefore, the hot sun emits much more radiation energy than the
cooler Earth and its peak emission is at a much shorter wavelength
than the Earth.
o See figure 2.9. Peak emission for the Sun falls into the
visible radiation category, while peak emission for the
Earth (and most objects on the Earth) is infrared radiation,
which we cannot see with our eyes.
 Photons
o At a very basic level, radiation energy is emitted photon by photon. Billions upon
billions upon billions … of photons. We have seen that different types of radiation
can be described by the wavelength of its electric and magnetic fields (shorter
wavelengths are said to carry more energy). Alternatively, different types of radiation
can be described by the energy carried by a single photon. A photon can be thought
of as a distinct “chunk” of electromagnetic radiation. It is the smallest amount of
radiation energy that can exist, i.e., photons cannot be broken down. This gives rise
to the concept that radiation energy is “quantized” or comes in discrete packages (as
photons). The radiation energy at any given wavelength is composed of a discrete or
integer number of photons. Keep in mind, thought, that there exists a continuous
spectrum of photon energies in the universe.
 Because only a small fraction of all the radiation energy emitted from the Sun
is in the form of ultraviolet radiation (figure 2.8), we can say that the Sun
emits many more visible photons than ultraviolet photons (equating to more
visible radiation energy than ultraviolet); however, each ultraviolet photon
carries more energy than each visible photon.
o At the very basic level, radiation energy is absorbed photon by photon as well. With
respect to human well-being, an important issue here is what happens to a living cell
when it absorbs a photon.
 An ultraviolet photon has enough energy to damage or destroy the DNA in a
living cell when it is absorbed.

A visible photon does not contain enough energy to damage the cell when it is
absorbed.

Thus the absorption of many visible photons does not cause a problem, but the
absorption of a single ultraviolet photon can cause problems. This is why we
must protect ourselves from exposure to ultraviolet radiation (photons).
End of Quiz 5 Material
Blue Skies, Red Sunsets, White clouds, and Haze (beginning Chapter 15)
 We will now briefly digress to discuss some common optical phenomena in the atmosphere
 When radiation energy hits an object, one of three things will happen
o The radiation can be transmitted through the object (pass through unaffected)
 Draw simple schematic diagram for transmission
o The radiation can be absorbed by the object. With absorption, the energy carried by
the radiation is taken in by the object. The absorbed energy may act to raise the
temperature of the object.
 Draw simple schematic diagram for absorption
o The radiation can be scattered by the object. Scattering changes the direction that the
radiation was moving. There is no energy absorption, just a re-direction of the
radiation energy
 Draw simple schematic diagram for scattering
 What actually happens (transmission, absorption, or scattering) depends on both the type of
material that the object is made of and the type (or wavelength) of the radiation that hits it.
o For example, sand behaves differently than water, both of which behave differently
than grass. The material snow will scatter most visible radiation that hits it, but will
absorb most infrared radiation that hits it.
 The human eye is sensitive to visible radiation. Visible radiation originates from objects hot
enough to emit visible light (e.g., the Sun, light bulbs, etc.). We are able to distinguish
different “colors” (wavelengths) of visible light. When our eyes are hit by nearly equal
amounts of all colors, we perceive the color white. For objects that do not emit visible light,
we “see” that object based on it scatters visible light.
o Draw a simple diagram to show this
 We will now consider what happens to visible radiation from the Sun that encounters the
Earth’s atmosphere. Some of it is transmitted, some is absorbed, and some is scattered.
o Draw a simple diagram to describe this
 All of the optical phenomena that we will describe here are the result of scattering. The
scattering is done by molecules of gas in the atmosphere, cloud droplets, and aerosols. To
first order, the properties of each scatterer depend on the size of the particle in relationship to
the size of the wavelength of the radiation. We will not explain those details, only give the
results …
o Clouds appear white because cloud droplets scatter all wavelengths of visible light
about equally.
 See figures 15.1 and 15.2.
 It may be easier to understand what is going on by thinking of white light as
being composed of individual photons of all visible colors. For cloud droplets
each different “color” of visible photon is equally scattered.
o The sky appears blue (when you look away from the Sun) because individual gas
molecules in the atmosphere scatter shorter wavelength visible light (blue) more
efficiently than longer wavelength visible light (red).
 Draw what happens when white light passes through a long tube of air.
 See also figure 15.4.
 You should realize that if visible light were not scattered by gas molecules
(and/or aerosols) in the atmosphere, the sky would be black. Only light on a
direct path from the Sun to your eyes would be seen … the sky away from the
sun would be black (would be able to see stars even during the day).
o During sunrise and sunset, the disc of the Sun often appear red. At those times of
day, sunlight travels through a long path in the atmosphere. What you see then is the
light which has been transmitted through a long path in the atmosphere. Since most
of the blue will be scattered out of this long path by gas molecules, the remaining
transmitted light is red.
 This was also shown on the previous diagram. Also see figure 15.8.
 During most of the day the disc of the Sun looks white. This is because it is
only when the sun is near the horizon that the path through the atmosphere is
long enough to scatter away enough blue light from the direct beam to make
the disc of sun look red.
o
Haze is an atmospheric phenomenon where aerosols (e.g., dust, smoke, sea salt, etc.)
obscure the clarity of the sky or affect visibility. The scattering and absorption
properties of individual aerosols depend on the size and composition of the aerosol.
 Some very interesting, but rare, optical phenomena can be caused by hazes
which scatter or absorb different colors of visible light differently. For
example some hazes scatter blue light more than red (like gas molecules),
while other hazes may scatter red light more than blue. We will not discuss
these rare cases.

Most often aerosols scatter (and/or absorb) all colors of light equally, so they
appear white. The most noticeable affect of haze is usually a reduction in
visibility wherein the sky takes on a whitish appearance. Many types of
aerosols swell or expand as relative humidity increases, so haze is often more
apparent in more humid climates.
 Draw diagram to show how haze limits visibility
 Explain “crepuscular rays” using figure 15.7.